An inexpensive current limiter for flashing and initial testing of ESCs – Mk I

There is a risk of damage when flashing ESCs. It accrues from the fact that ESCs have a three legged H bridge and if a high and low FET are turned on simultaneously, damaging currents may flow. In fact, this can be an issue if the FETs are on together for just microseconds on each PWM cycle. Loading the wrong hex module is a recipe for disaster, it may turn on FETs in an unexpected way.

So, for safety, the ESC should be powered from a current limited power supply during flashing and initial motor testing.

The article describes a low cost current limiter based on an incandescent lamp and a diode.

The idea is to make up a little adapter to go between your ESC and Lipo battery. The adapter goes in series between the battery + and the ESC +, and it is a 12V incandescent lamp and a high current diode in parallel, diode cathode towards the battery.


Above is a prototype limiter lashed up as proof on concept.

Battery – is wired directly to ESC -.

The lamp limits the current that the ESC can draw from the battery. In my case, I used an 18W 12V lamp and I could draw up to 0.5A before voltage to the ESC fell too much. Maximum current on 3S is about 1.3A into a short circuit. If you have loaded the wrong hex module, and it switches a high and low FET on, it will probably sustain 1.3A without getting warm, much less being damaged.

The diode is to provide a current path for regenerative current flowing back to the battery, and it ensures that the ESC + terminal cannot go higher than about 1V above the battery + voltage (protecting the FETs and regulators from overvoltage). The diode I used is a twin Schottky diode rated at 25A each, and 50V breakdown, you can get them on eBay for a few dollars.

Don't get the diode back to front!!!

Some people will no doubt insist that the diode is not needed. I would advise that it is a good idea to include it, especially if you use COMP_PWM or any form of regenerative braking. If you aren't sure, then use the diode.

From a 3S battery, I was able to power the ESC for programming, and run a Turnigy 2730-1500 motor up to 5000 RPM for testing, about 0.5A current draw. This is enough to be confident that the motor starts and runs properly, and that FETs on the ESC are not overheating due to shoot through current.

I usually use an electronic regulated supply on the bench, but this little device can be taken into the field and flash updates done from a notebook computer and safely tested with the current limiter, the normal Lipo battery and a servo tester.